SEARCH · Engineering Papers
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
The temperature of Neptune's exosphere is predicted, using Voyager observations of the exospheric temperatures of the outer planets. The exospheric temperature at Neptune is estimated at about 200 K, compared with about 800 K at Uranus. The implictions of this work for the existence of a Neptune electroglow are discussed.
This paper offers a prediction concerning Neptune's low-frequency radio emission based on the radiometric Bode's law in combination with a recent prediction for Neptune's global magnetic field strength. The latter is based on a dynamo scaling relation derived from the magnetospheric balance condition within planetary cores. The radio emission frequency range is predicted to extend from approximately 100 to 1000 kHz, with a spectral peak between 350 and 500 kHz. A crude estimate of the emission spectral shape, based on Saturn and earth-like models, is shown. If radiation is beamed approximately in the sunward direction, Neptune should be detectable by the Planetary Radio Astronomy experiment onboard the Voyager spacecraft sometime between 45 and 60 days before closest approach.
Digital images of Neptune showing cloud structure at visible wavelengths were obtained in July 1988. A discrete bright feature was detected both at 6190 A (a weak methane absorption band in the visible) and at 8900 A (a stronger methane band in the near infrared). The images also revealed that Neptune's southern pole was bright relative to planetary mid-latitudes at 6190 A but not 8900 A. The implications of these findings for atmospheric rotation and structure are discussed. The detection of discrete features at visible wavelengths is of special importance to the upcoming Voyager encounter with Neptune: the wide-angle camera has a 6190 A filter similar to that used for these observations.
Spacecraft observations of the seven innermost planets suggest that lightning is present on all planets with dense atmospheres. Thus lightning may be expected to occur on Neptune as well. Based on the level of lightning activity observed on Jupiter and on the assumption that the lightning energy dissipation rate is a constant fraction of the dissipation rate of the convective energy, the lightning rate on Neptune is estimated to be 1/19 that observed on Jupiter. It is predicted that approximately 60 events should be recorded by the Voyager imager during its search of the nightside of Neptune.
Perturbations from Neptune's highly inclined satellite Triton can maintain rings passing nearly over Neptune's poles. These hypothetical polar rings are nearly perpendicular to Triton's orbit as well, and lie within several degrees of the plane of Voyager II's trajectory through the Neptunian system. Polar rings can coexist with equatorial rings at different radii. A randomly oriented torus of debris around Neptune has a probability of several percent to settle into a polar ring. Voyager II stands a significant chance of encountering a polar ring.
This paper explores some possibilities for plasma populations, field-aligned currents, and field-aligned potentials in the magnetosphere of Neptune. Observed plasma populations at Saturn and Uranus may provide reasonable upper and low limits, respectively, to those at Neptune. Field-aligned current densities comparable to those at Earth may be observed by the Voyager magnetometer above Neptune's auroral zone. Inverted-V events reaching energies of several tens of keV may also be observed by the Voyager Low Energy Charged Particle experiment.
Triton, which is currently spiralling toward Neptune due to tides raised on both bodies, possesses an obliquity which may lie close to either a zero-deg 'state 1' or a 100-deg 'state 2' which correspond to the two stable Cassini extrema of its rotational Hamiltonian. The Kaula (1966) tidal formalism is presently used to model the past and future evolution of the system in both states. For nominal parameters in state 1, Triton will reach Neptune's Roche limit in about 3.6 Gyr with a decrease in orbital inclination to 145 deg from the current 159 deg; in the case of state 2, Triton's inclination will increase to 180 deg in 10-100 million years and then transition to state 1, subsequently reaching the Neptune Roche limit in about 1.4 Gyr.
Ground-based CCD imaging of Neptune in the near-IR clearly shows discrete cloud features. The brightest cloud features on Neptune are confined to latitudes from about 30 to 50 deg (in both hemispheres). Fainter enhanced regions are sometimes seen at even higher latitudes. Imaging obtained prior to 1986 showed bright clouds in both northern and southern midlatitudes, giving Neptune the appearance of having a dark equatorial belt. But since 1986, only the southern hemisphere has shown clouds; no bright features have been seen in the northern hemisphere. The equatorial region is not significantly darker than the northern midlatitudes. Discrete features are most obvious in the strongest methane band (8900 A), but brightness asymmetry is seen in weaker methane bands (7270 and 6190 A). Disk-integrated photometry obtained from the 1986 and 1987 imaging was calibrated absolutely with the same flux standard. In both years, variability at 8900 A was dominated by scattered light from a single bright cloud feature. The amplitude of the variability was a factor of two higher in 1987. The brightness of the less active hemisphere appears relatively constant between the two years, although some variability is seen on this hemisphere in 1987.
Results from the occultation of the sun by Neptune imply a temperature of 750 + or - 150 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane, acetylene, and ethane at lower levels. The ultraviolet spectrum of the sunlit atmosphere of Neptune resembles the spectra of the Jupiter, Saturn, and Uranus atmospheres in that it is dominated by the emissions of H Lyman alpha (340 + or - 20 rayleighs) and molecular hydrogen. The extreme ultraviolet emission in the range from 800 to 1100 angstroms at the four planets visited by Voyager scale approximately as the inverse square of their heliocentric distances. Weak auroral emissions have been tentatively identified on the night side of Neptune. Airglow and occultation observations of Triton's atmosphere show that it is composed mainly of molecular nitrogen, with a trace of methane near the surface. The temperature of Triton's upper atmosphere is 95 + or - 5 kelvins, and the surface pressure is roughly 14 microbars.
Voyager 2 undertook radio science investigations of the Neptune and Triton masses and densities, as well as of their atmospheric and ionospheric vertical structures, the atmospheric composition and low-order gravitational harmonics of Neptune, and ring material characteristics. Upon probing the atmosphere of Neptune to a pressure level of about 500,000 Pa, the effects of a methane cloud region and of ammonia absorption below the cloud have become apparent. The tenuous neutral atmosphere of Triton produced distinct signatures in the occultation data; it is inferred that the Triton atmosphere is controlled by water-pressure equilibrium with surface ices.
The Voyager 2 magnetic field experiment discovered a complex and powerful magnetic field in Neptune, as well as an associated magnetosphere and magnetic tail. As the spacecraft exited the magnetosphere, the magnetic tail appeared to be monopolar. The auroral zones are probably located far from the rotation poles, and may possess complex geometry. The Neptune rings and all its known moons are imbedded deep within the magnetosphere (except for Nereid, which is outside when it lies sunward of the planet); the radiation belts have a complex structure due to the absorption of energetic particles by the moons and rings of Neptune, as well as losses associated with the significant changes in the diurnally varying magnetosphere configuration.
The results of Voyager 2 observations of Neptune are reviewed. Observations of Neptune's Great Dark Spot, rotation atmosphere, magnetic field, rings, and satellites are discussed. Also, observations of Triton are considered, noting the presence of geyser activity on the satellite. Several photographs of features on both Neptune and Triton are presented.
The present observational results for Neptune's S3(1) and S4(1) H2 lines show the former line's measured equivalent width to be the same as for this feature in Uranus, repeating the equality already established between the two planets for the latter feature. It is also noted that the observed ratio of the H2 S3(0)/S3(1) lines for Neptune's atmosphere is reproduced by models belonging to the family of models created by Baines and Smith (1990); by comparison with the earlier Uranus models of Baines and Bergstrahl (1986), the greater continuum absorption of Neptune is responsible for the increased S3(0)/S3(1) line ratio near 0.82 microns.
The 3nu2 band of CH3D was detected in the spectrum of Neptune near 1.6 micron recorded at a spectral resolution of 4/cm with the Cassegrain Fourier Transportation Spectrometer at the 3.6 m Canada-France-Hawaii Telescope CFHT) on Mauna Kea. The analysis of this spectrum, using spectral synthesis techniques, yielded a CH3D/CH4 ratio of about 0.0006, which corresponds to a global D/H ratio for Neptune of about 0.00012, if CH3D is in isotopic fractionation equilibrium with HD. This value is about an order of magnitude larger than an earlier estimate by Orton et al. (1987) based on deconvolution measurements of unresolved molecular emission in the 8-10-micron region. Comparison of this new determination with previous studies of CH3D in the outer solar system shows that, as in the case of Uranus, the D/H on Neptune is strongly enhanced over that found on Jupiter and Saturn and is comparable to the D/H in methane on Titan and in terrestrial methane and water.
This article presents the results of a fit of numerically integrated Neptunian satellite orbits to earth-based astrometric observations and early Voyager spacecraft observations. Ephemerides based on these orbits were used by the Voyager project as the final pre-encounter ephemerides. As a by-product of the orbit fits, estimates of the Neptune mass, the second zonal harmonic of Neptune, and the pole orientation of Neptune were also obtained.
Triton is shown to be the dominant source of plasma for L equal to or greater than 7 in the magnetosphere of Neptune. Triton maintains a neutral hydrogen torus of average density comparable to a greater than that of the Titan torus at Saturn. The Triton torus may be detectable in H Lyman-alpha emissions. However, the energy source from plasma outward transport and mass loading in the Triton torus is insufficient to explain the Neptune aurora. It is proposed that Neptune's aurora is driven mainly by a solar wind interaction.
This paper presents values of particle spectral parameters and estimates of plasma densities, temperatures, and beta parameters, obtained with the Low Energy Charged Particle instrument during the Voyager 2 encounter with Neptune on August 24-25, 1989. In addition, trapped electron intensities are compared with the whistler mode stably trapped limits. The results revealed a very good inbound-outbound symmetry for both the proton and the electron profiles, suggesting that there was little dynamical activity in Neptune's magnetosphere during the Voyager encounter. The similarities and differences observed between Neptune and Uranus in the values of plasma density, pressure, and beta are discussed.
The Voyager 2 Ultraviolet Spectrometer measured the emission intensity of the He resonance line at 584 A to be 0.34 + 0.2 or - 0.15 R on the day side of Neptune. Calculations of the He 584 A intensity at Neptune using partial frequency redistribution and inhomogeneous atmospheric models show that the product of the volume mixing ratio (mole fraction), fHe, and the eddy diffusion coefficient at the homopause, Kh, is fHeKh = 10 to the 7th sq cm/s with upper and lower bounds to the uncertainty of about a factor of 3 and 9, respectively. If fHe is taken as the current Infrared Interferometer Spectrometer working value, fHe = 0.19, then Kh = 5 x 10 to the 7th sq cm/s with a similar uncertainty. This range of K overlaps that obtained from analysis of the hydrocarbon distributions on Neptune (Broadfoot et al., 1989).